Terminology 101: Confidence intervals: Part 2

May 2012   Comments

Confidence interval: The range of values, consistent with the data, that is believed to encompass the actual or “true” population value

Source: Lang, T. A. & Secic, M. (2006). How to Report Statistics in Medicine. (2nd ed.) Philadelphia: American College of Physicians

Part 1, which appeared in the February 2012 issue, introduced the concept of confidence intervals (CIs) for mean values. This article explains how to compare the CIs of two mean scores to draw a conclusion about whether or not they are statistically different. Two mean scores are said to be statistically different if their respective CIs do not overlap. Overlap of the CIs suggests that the scores may represent the same “true” population value; in other words, the true difference in the mean scores may be equivalent to zero. Some researchers choose to provide the CI for the difference of two mean scores instead of providing a separate CI for each of the mean scores. In that case, the difference in the mean scores is said to be statistically significant if its CI does not include zero (e.g., if the lower limit is 10 and the upper limit is 30). If the CI includes zero (e.g., if the lower limit is -10 and the upper limit is 30), we conclude that the observed difference is not statistically significant.

To illustrate this point, let’s say that we want to compare the mean blood pressure (BP) of exercising and sedentary patients. The mean BP is 120 mmHg (95% CI 110-130 mmHg) for the exercising group and 140 mmHg (95% CI 120-160 mmHg) for the non-exercising group. We notice that the mean BP values of the two groups differ by 20 mmHg, and we want to determine whether this difference is statistically significant. Notice that the range of values between 120 and 130 mmHg falls within the CIs for both groups (i.e., the CIs overlap). Thus, we conclude that the 20 mmHg difference between the mean BP values is not statistically significant. Now, say that the mean BP is 120 mmHg (95% CI 110-130 mmHg) for the exercising group and 140 mmHg (95% CI 136-144 mmHg) for the sedentary group. In this case, the two CIs do not overlap: none of the values within the first CI fall within the range of values of the second CI. Thus, we conclude that the mean BP difference of 20 mmHg is statistically significant.

Remember, we can use either the CIs of two mean scores or the CI of their difference to draw conclusions about whether or not the observed difference between the scores is statistically significant.


NurseONE resources on this topic

EBSCO-Medline Full-text articles

  • Hildebrandt, M., Vervölgyi, E., & Bender, R. (2009). Calculation of NNTs in RCTs with time-to-event outcomes: A literature review. BMC Medical Research Methodology, 9, 21.
  • Hildebrandt, M., Bender, R., Gehrmann, U., & Blettner, M. (2006). Calculating confidence intervals for impact numbers. BMC Medical Research Methodology, 6, 32.
  • Altman, D. G. (1998). Confidence intervals for the number needed to treat. BMJ (Clinical Research Ed.), 317(7168), 1309-1312.

MyiLibrary

  • Campbell, M. J., Machin, D., & Walters, S. J. (2010). Medical statistics: A textbook for the health sciences (4th ed.).
  • Mateo, M. A., & Kirchhoff, K. T. (Eds.). (2009). Research for advanced practice nurses: From evidence to practice.
  • Webb, C., & Roe, B. (Eds.). (2007). Reviewing research evidence for nursing practice: Systematic reviews.
Maher M. El-Masri, RN, PhD

Maher M. El-Masri, RN, PhD, is a full professor and research chair in the faculty of nursing, University of Windsor, in Windsor, Ont.

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